The current patch gives much smaller benefits than the originally posted benchmarks, although they are still substantial:

$ ./python -m timeit -s "a = set(range(100000)); sd = a.difference; b = set(range(1000))" "sd(b)"
- before: 5.56 msec per loop
- after: 3.18 msec per loop

$ ./python -m timeit -s "a = set(range(1000000)); sd = a.difference; b = set(range(10))" "sd(b)"
- before: 67.9 msec per loop
- after: 41.8 msec per loop